The catalytic dehydration of hydrocarbons according to the formula:CnH2n+2⇄CnH2n+H2  (1)which as a rule takes place in the gaseous phase at a temperature between 540° C. and 820° C., is a highly endothermic equilibrium reaction the conversion rate of which is limited by the thermodynamics and depends on the respective partial pressures and the temperature. A dehydration reaction is facilitated by low partial pressures of the hydrocarbons and high temperatures. The side reactions leave cracked products, which form hydrocarbon deposits and cause a catalyst deactivation so that the catalyst must be periodically regenerated during plant operation.
If the dehydration takes place in an adiabatically operated catalyst bed, the endothermic reaction entails a gradual drop in temperature over the length of the catalyst bed. Hence, the conversion rate obtained in the catalyst bed is restricted so that the high rates envisaged necessitate several catalyst beds and re-heating downstream of each catalyst bed.
A catalytic dehydration of paraffins to obtain olefins may also take place in a heated, i.e. isothermic catalyst bed. U.S. Pat. No. 5,235,121, for example, describes a process which uses an input mixture consisting of light paraffins and water vapour and being fed to a tubular reactor with external firing system, i.e. the catalyst bed is a heated fixed bed. The catalyst used is designed in such a manner that a steam reforming process cannot take place in the presence of water vapour, i.e. no reaction of hydrocarbons with water vapour, forming CO, CO2 and H2. The catalyst is regenerated in cyclic intervals. Patent DE 198 58 747 A1 describes a similar process.
Heating the catalyst bed or the isothermic mode of operation (MO) permit very high conversion rates in a bulk catalyst bed. This MO, however, involves a disadvantage because the said rates, which are due to the level of the thermodynamic equilibrium, can be obtained at very high temperatures only, a fact that reduces the selectivity of olefin formation.
The a/m MO in the presence of water vapour bears the advantage that the partial pressure of the hydrocarbons is reduced by the water vapour and thus contributes to a higher conversion rate. Moreover, the utilisation of water vapour permits the conversion of a certain part of hydrocarbons to CO2 on the catalyst so that the cycle times, i.e. operating periods of dehydration between the regeneration cycles, can be prolonged. But it is true that too large quantities of water vapour are detrimental to the process as there is a substantial increase in the gas stream volume which entails additional investment costs and impedes the process efficiency. Furthermore this increases the risk of steam reforming of hydrocarbons, which leads to the loss of product and/or reduces the yield. The quantity of steam the can be added without causing the problems described above depends on the absolute pressure applied in the reaction process and on the dehydration catalyst itself.
There is a further possibility of overcoming the thermodynamic limitation of the conversion at equilibrium, i.e. by feeding oxygen to selectively burn a portion of the hydrogen obtained by dehydration in accordance with2 H2+O2→2 H2O  (2)—a process step also named “SHC” short for “Selective Hydrogen Combustion”—so that the equilibrium of the dehydration is re-arranged such that higher conversion rates become feasible. Patent EP 0 799 169 B1 describes a reactor for such a dehydration process with SHC, a mixture of paraffin and oxygen being sent via a first catalyst which performs dehydration and selective oxidation of the hydrogen obtained, a further admixture of oxygen taking place in an intermediate reactor chamber and finally, there is a second catalyst that also performs dehydration and selective oxidation of the hydrogen obtained. The process described in EP 0 799 169 B1 takes place in an autothermal mode and the highly exothermic reaction (2) of the hydrocarbon with oxygen supplies the energy required to carry out the endothermic dehydration reaction (1).
Patent WO 96/33150, for example, describes a process in which the paraffin mixture is first dehydrated in a primary step, oxygen then being added and this oxygen reacting at least in a secondary step with the hydrogen released by dehydration so that water vapour is obtained. At least a part stream of the product obtained undergoes a secondary dehydration in order to dehydrate the paraffins not yet converted; recycling of a part stream to the primary step is also suggested.
The disadvantage of the process is that the addition of oxygen and the exothermic oxidation of hydrogen lead to very high temperatures, which reduces the selectivity of the downstream catalytic dehydration. This particularly applies to isothermic dehydration in the first step because in the case of adiabetic dehydration carried out in the first step, the temperature drop occurring in the catalyst bed causes an input temperature towards the SHC step that is lower than that in the case of isothermic dehydrations.
The problem of superheating due to hydrogen oxidation can be solved by intermediate cooling that takes place before the selective hydrogen oxidation and thus reduces the temperature at the inlet of the second catalyst bed. U.S. Pat. No. 4,599,471, for example, suggests such an intermediate cooling either of the direct or indirect type. The direct cooling is feasible with the aid of inert gas such as nitrogen, helium, etc. or steam.
Temperature control through indirect cooling, however, has a disadvantage because it requires firmly installed heat exchangers, which does not permit specific temperature guidance for catalyst bed regeneration, or it necessitates a circuitry for intermittent uncoupling of the heat exchanger, for example, by means of a by-pass that can be shut down by additional valves. This configuration, however, would be extremely expensive on account of the large pipe cross-sections and the high operating temperatures of approx. 500-650° C. due to the dehydration.
Direct cooling with the aid of inert gases bears a disadvantage because later they must be separated from the product in expensive steps in order to further process the product. Indirect cooling with the aid of steam has a disadvantage since in the reaction as described above is not inert and develops a certain steam-to-hydrocarbon ratio depending on the cooling. Hence, the steam quantity considerably increases with the cooling intensity, which is detrimental to the process, i.e. it becomes evident through an increase in the construction scope and the probability of an undesired steam reforming of hydrocarbons.